Chem question

Aqueous hydrogen ions (H^+), which is responsible for the acidity of the solution, can turn blue litmus paper red. Aqueous hydroxide ions (OH^-), which is responsible for the alkalinity of the solution, can turn red litmus paper blue.

Ammonia gas has molecular structure which contains discrete NH3 molecules, but does not contain hydrogen ions or hydroxide ions. Therefore, pure (dry) ammonia gas cannot turn red litmus paper to blue, and it also cannot turn blue litmus paper to red.

When the red litmus paper is moistened, ammonia gas is dissolved in water (which is used to moisten the litmus paper) and is then partly ionized in water to give aqueous hydroxide ions.
NH3(aq) + H2O(l) = NH4^+(aq) + OH^-(aq)
("=" represents the double arrows sign for reversible reaction.)
The aqueous hydroxide ions turn the red litmus paper blue.

NH3(aq) + H2O(l) = NH4^+(aq) + OH^-(aq)
At the equilibium, the concentration of NH4^+(aq) is not very high, so by increase the concentration of OH^-(aq) through the addition of water, so the dissoultion of NH3 is better and the result is better.

"ionize the litmus paper" ?
嘩！好得人驚呀！

Moisten the paper to ioniz the litmus paper.
The litmus paper can only determine the acidicity in molten stage.
NH3 + H2O <-> NH4+ + OH-